I was told by my avionics shop that the Garmin portables can only receive input from other devices (430/530 etc) and have no output capability. So there will be no way to get the weather or anything else out to a larger screen.

The new Gamin 510 has a section in the manual that talks about the input and out capabilities- such as TIS but has no interface for weather. The manual has a diagram of the cable with accepted input and output formats.